There is a continuing need to improve the recording density of data storage devices. Such data storage devices include magnetic storage devices, such as magnetic disk drives. The use of thin-film magnetic heads, such as a composite thin-film magnetic head, and higher-performance magnetic recording media has enabled some level of improvement in storage capacity. A thin-film magnetic head may stack, on a substrate, a read head, including a magnetoresistive element (hereinafter also referred to as MR element), and a write head, including an induction-type electromagnetic transducer. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
Magnetic recording media used in magnetic recording devices, such as hard disk drives, are made of an aggregate of magnetic fine particles, and each bit is recorded using more than one magnetic fine particle. Recording density may be improved by reducing asperities at the borders between adjoining recording bits, which can be achieved by making the magnetic fine particles smaller and using a correspondingly-smaller write head. But decreasing the asperities at the borders between adjacent recording bits causes the thermal stability of magnetization of the magnetic fine particles to decrease with decreasing volume of the magnetic fine particles. To mitigate this problem, the anisotropic energy of the magnetic fine particles may be increased, but doing so leads to an increase in coercivity of the magnetic recording medium, which increases the difficulty of writing data. This problem is exacerbated because it can be difficult to generate a magnetic field having a sufficient magnitude using a small write head.
Heat-assisted magnetic recording (HAMR), also referred to in the art as thermally-assisted magnetic recording (TAMR) or energy-assisted magnetic recording (EAMR), has been developed to allow the use of smaller write heads with higher-coercivity magnetic recording media to improve areal density capacity. HAMR uses heat to lower the effective coercivity of a localized region on the magnetic media surface and writes data within this heated region. The data state becomes “fixed” upon the media cooling to ambient temperatures. Thus, in HAMR, a magnetic recording material with high magneto-crystalline anisotropy (Ku) is heated locally during writing to lower the coercivity enough for writing to occur, but the coercivity/anisotropy is high enough that the recorded bits are thermally stable at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature of approximately 15-30 degrees Celsius). The recorded data may then be read back at ambient temperature by a conventional magnetoresistive read head. HAMR disk drives have been proposed for both conventional continuous media, wherein the magnetic recording material is a continuous layer on the disk, and for bit-patterned media (BPM), in which the magnetic recording material is patterned into discrete data islands or “bits.”
One type of HAMR disk drive uses a laser source and an optical waveguide coupled to a near-field transducer (NFT) for heating the recording material on the disk. The laser source may be a laser diode of InP type, GaAs type, GaN type, or the like, such as used in applications such as communications, optical disc storage, and material analysis. The laser source may emit laser light of any wavelength within the range of, for example, 375 nm to 1.7 μm. The laser source may be located on the slider or in a remote location. The waveguide may be made from any suitable material. For example, the waveguide may be polymer, quartz fiber, or plastic fiber.
A near-field transducer refers to “near-field optics,” wherein light is passed through a first element with subwavelength features and the light is coupled to a second element, such as a substrate (e.g., of a magnetic recording medium), located a subwavelength distance from the first element. The NFT is typically located at the air-bearing surface (ABS) of an air-bearing slider that also supports the read/write head and rides or “flies” above the disk surface. A NFT may have a generally triangular output end, such that an evanescent wave generated at a surface of the waveguide couples to surface plasmons excited on the surface of the NFT, and a strong optical near-field is generated at the apex of the triangular output end. The NFT couples light onto the media at a spot of a size that is smaller than the optical diffraction limit, which heats a region of the media.
Typically, NFTs have two features: a large plasmon resonator made of a plasmonic metal (e.g., gold) that generates near-field light from plasmons excited by irradiation with light, and a smaller-scale structure, also made of a plasmonic metal, that creates a localized heating of the media by coupling the electromagnetic energy stored in the antenna to the media passing below the NFT. The plasmon resonator has a size that is less than or equal to the wavelength of the light being used to heat the media.
To write data, a magnetic field and heat are simultaneously applied to the area of the magnetic recording medium in which data is to be written. As a result, the temperature of the area increases and the coercivity decreases, thereby enabling the data to be written at a relatively modest field. In order to prevent unintended writing or erasing, the spot diameter of irradiated light should approximately match the size of a recorded bit.
A drawback of a NFT that generates near-field light by direct irradiation with light is the low efficiency of transformation of the applied light into near-field light. Most of the energy of the light applied to the NFT is lost, either by reflecting off the surface of the NFT or by being transformed into thermal energy and absorbed by the NFT. Because the NFT is small in volume, the temperature of the NFT can increase significantly when it absorbs the thermal energy. This temperature increase can cause the NFT to expand in volume and/or deform.
FIG. 1A illustrates a cross-section of a prior-art NFT 130 in which a metal layer 134 is encased within an insulator layer 132 in a construct known in the art as the “insulator-metal-insulator” or “IMI” configuration. Because plasmonic metals have higher thermal expansion coefficients than dielectrics, as the NFT 130 heats up, the mismatch of thermal expansion coefficients between the metal layer 134 and the encasing insulator layer 132 creates high pressure, which causes plastic deformation or “flow” of the plasmonic metal. (As would be appreciated by a person having ordinary skill in the art, a material's thermal expansion coefficient describes how the size of an object made from the material changes with a change in temperature. Specifically, the thermal expansion coefficient characterizes the fractional change in size per degree change in temperature at a constant pressure.) The amount of pressure developed is proportional to the product of (i) the mismatch of the coefficients of the thermal expansion, (ii) temperature, and (iii) the volume of the plasmonic metal in the NFT. The temperature increase in an IMI implementation may cause a large, sharp pinpoint protrusion of the NFT 130 from the ABS 160 and toward the media passing below the slider, as shown in FIG. 1B. If the slider fly height is not adjusted to account for the protrusion, the protrusion may touch the magnetic recording medium, potentially shearing off and/or causing damage to or failure of the magnetic recording device. Alternatively, the protrusion can result in the slider having to fly at a larger distance from the magnetic recording medium than the optimal height, which may adversely affect the ability of the read head to read data on the magnetic recording medium.
An example of an IMI NFT is the so-called “lollipop” NFT, which has an enlarged disk-shaped region as the large plasmonic resonator and a peg as the smaller-scale structure. The tip of the peg, at the slider ABS, may be covered in a thin layer of diamond-like carbon (DLC). In lollipop NFTs, the enlarged disk-shaped region receives concentrated light through the waveguide and is designed to help the NFT achieve surface plasmon resonance in response to this concentration of light. The disk-shaped region typically comprises most of the volume (e.g., between 90% and 95%) of the NFT. The peg is in optical and/or electrical communication with the disk-shaped enlarged region and creates a focal point on the media for the energy received by the enlarged region. Because the disk-shaped region is large in comparison to the peg, and the disk-shaped region is encased in an insulator that does not expand at the same rate as the plasmonic metal of the disk-shaped region, temperature increases of the disk-shaped region cause the smaller peg to expand in a way that is relatively dramatic. For example, the pressure developed because of the mismatch of thermal expansion coefficients between the plasmonic metal and the encasing insulator may cause the peg to elongate, potentially breaking the DLC protective layer at the ABS. In addition or instead, the peg may protrude, temporarily or permanently, toward the media as the disk-shaped region temperature increases and then retreat away from the media as the disk-shaped region's temperature decreases. These deformations of the peg can reduce the effectiveness of the NFT and the performance of the HAMR device. They may also lead to failure of the magnetic storage device or shorten its life considerably.
FIG. 2A illustrates another NFT implementation 140, in which an insulator layer 144 is encased in a metal layer 142 comprising a plasmonic metal in a configuration known in the art as a “metal-insulator-metal” or “MIM” configuration. As shown in FIG. 2B, temperature increases may cause the metal layer 142 of the MIM NFT 140 to protrude from the ABS 160 in a more moderate and smooth way. Even in this configuration, however, repeated deformation of the NFT can adversely affect the expected life of the magnetic recording device.
There is, therefore, a continuing need for improved NFT designs that control NFT deformations better than prior-art designs.